1. Technical Field
The invention relates to a collector used in, for example, alkaline secondary batteries, and to a battery electrode substrate that uses the collector.
2. Background Art
The collectors developed so far and used in alkaline secondary batteries or the like include the following.
For example in Patent Document 1, a metallic porous body obtained by the three-dimensional entanglement of metal fibers is applied to the collector for an alkaline secondary battery. The metal fiber diameter, pore diameter, porosity, and density optimal for the collector are specified therein. Within the sphere of methods for fabricating porous metal structures, Patent Document 2 discloses a method in which a conductive layer is formed on a polymer core material such as unwoven fabric by a vapor-phase method such as sputtering, followed by formation of a metal layer by electroplating. In Patent Document 3, unwoven fabric material is subjected to a surface treatment such as sulfonation, and a nickel plating film is then formed thereon; this is used as a collector material for alkaline secondary batteries. It is described that flexibility and strength are secured by letting a polymer unwoven fabric remain as the core material. Patent Document 4 discloses a high-capacity collector material capable of high-rate charge/discharge, which is obtained by specifying the amount of plating on the surface of unwoven fabric in terms of the cross-sectional area. Patent Document 5 discloses a collector material capable of high-rate charge/discharge, which is obtained by specifying the thickness of unwoven fabric material, and the method of fabrication.
Patent Document 1: Japanese Patent Publication No. H2-216766
Patent Document 2: Japanese Patent Publication No. S61-76686
Patent Document 3: Japanese Patent Publication No. 2001-313038
Patent Document 4: Japanese Patent Publication No. 2003-109600
Patent Document 5: Japanese Patent Publication No. 2003-282066
The collectors described in Patent Documents 1 and 2, however, exhibit an unsatisfactory collector strength and flexibility. They are also expensive due to their use of large amounts of Ni. The prior art in Patent Documents 1 and 2, being composed of only metal fiber, requires an increase in the amount of metal in order to secure strength; however, the increased amount of metal results in a loss of flexibility and protrusion of the metal fiber through the separator, causing short circuiting. The cost is also driven up due to the large amounts of expensive Ni metal. A reduction in the amount of metal in pursuit of lower costs not only results in deficient strength, but also in an increase in electrical resistance and impaired high-rate charge/discharge characteristics.
The prior art described in Patent Documents 3 to 5 first of all suffers from a high electrical resistance. It also exhibits a reduced cycle life due to inadequate film adhesiveness. The high electrical resistance in this prior art is understood to be caused by deficient coverage on the fiber surface by the metal film. An inexpensive substrate is obtained if amount of Ni plating on the unwoven fabric surface is smaller. In this case, however, the small amount of Ni on the unwoven fabric surface results in impaired current flow, increase in the electrical resistance, and reduction in the output characteristics. The collection performance is also diminished by film exfoliation caused by the expansion and shrinkage associated with repetitive charging and discharging, which results in reduced battery cycle characteristics. Another problem has been the substantial increase in the resistance of the substrate because of winding. The unwoven fabric is bent by winding, which causes exfoliation of the metal layer plated on the surface and increase in electrical resistance.